1. Field of the Invention
This invention relates to the production of grain-oriented silicon steel having very low core losses in such steel by boron infusion and heat treatment after final texturizing annealing.
2.Description of the Prior Art
There has been a long history in the steel industry of the production of steel containing 2.5 to 4% of silicon for electrical purposes. The production of such steel includes one or more cold rolling reductions with intermediate annealing if more than one cold reduction is practiced, and then the steel is final texture annealed to develop a desired grain-oriented texture. The grain-oriented texture is associated with obtaining lower core-loss values when the electrical-steel product is subsequently used, as, for example, to make a wound-core transformer or a stacked core transformer.
The matters of principal concern to producers and users of oriented silicon steel are electrical permeability and core loss properties and production cost. Efforts to yield favorable electrical properties include defining particular compositions to the steel, conditions for the rolling, annealing, compositions of and the manner of application of the separating-medium coating applied before final texturizing annealing. In recent years a reduction to the gage occurred for regular oriented silicon steel, i.e., steel having an induction greater than 1.870 tesla at 8 ampere turns per centimeter, and a relatively low core loss, such as not more than 0.720 watts per pound at 17 kilograms and 60 cycles per second, which corresponds roughly to 0.510 watts per pound at 15 kilogauss and 60 cycles per second, as referred to in some of the older references in the art. As the gage of the steel is reduced there is a tendency for the grain size to increase which is generally accompanied by an increase in the domain size. This tends to increase the eddy-current component of the electrical losses deriving from the domain wall motion which partly offsets the decrease in the classical eddy-current losses accrued from the gage reduction.
It is known in the art of making grain-oriented silicon steel to use steels containing a small amount of boron; see U.S. Pat. No. 3,905,842 and U.S. Pat. No. 4,096,001. This prior art teaches the inclusion of both boron and nitrogen in small quantities in the steel, as it is melted, to promote secondary recrystallization during the final texture-developing anneal.
As disclosed, for example, in U.S. Pat. No. 4,096,000, it is known in the art to provide an anneallng separator for silicon steel sheets which contains more than 90% by weight of magnesium oxide and between 0.01 to 2.0% by weight of B.sub.2 O.sub.3. There are numerous other patents which teach the application of a coating to the steel before the final texturizing anneal which contains, in most cases, a major portion of magnesium oxide and a minor amount of a compound of boron. Other patents disclosing annealing separators include U.S. Pat. Nos. 4,096,001 and 4,116,730 and their British counterparts, Nos. 1578911 and 1578912; as well as U.S. Pat. Nos. 3,700,506; 4,160,681; 4,179,315; and 4,200,477. However, a boron-containing material was always applied before, never after the final texturizing anneal. Slurries used as an annealing separator applied before the texture anneal serve several important functions: (1) a separating medium to prevent welding of coil wraps, (2) a reactant with SiO.sub.2 on the steel surface to form forsterite, (3) a reservoir for impurities and (4) a source of elements or compounds that in one way or another provide for an improved secondary recrystallization by interaction with the steel during texture formation. The foregoing patents all disclose recipes for MgO slurry designed to accomplish the above functions more effectively, and are often designed to interact with specific elements in the steel to be texture annealed.
After the final texturizing anneal of the steel to improve the observed core-loss values, it is known to apply a tensile-stress-inducing coating and/or to apply laser scribing to reduce the 180 degree domain wall spacing. Although laser scribing often yields excellent core-loss values, it is expensive to practice, and the benefits of the scribing operation are lost when stress-relief annealing of the textured steel is carried out for use in a core transformer.